The invention relates to analog-to-digital converters. The invention relates specially to calibrating an analog-to-digital converter.
Telecommunications is one of the fastest-growing business areas and therefore an ever increasing amount of data is transferred through mobile and fixed telecommunication networks. Telecommunication systems are digital, but the world around them is analog; therefore we need analog-to-digital converters (ADCs). When we talk we produce an analog signal, and the voice we hear is also analog. In addition, electromagnetic radiation, which carries digital signals through the air from a mobile phone to a base station, is analog. However, these analog signals are processed in the digital format, because. the digital world is more tolerant for interference from various error sources. Another reason for processing signals in the digital format is that design automation of the digital circuits enables us to design more complex digital circuits.
The ADC is a device which converts a continuous analog signal into a discrete and quantified digital signal. The throughput of the ADC often limits the throughput of the digital system operating on analog inputs. Throughput is the number of conversions processed in a given period. The rate of information transmission cannot exceed the channel capacity (C). The maximum channel capacity can be calculated by applying the Harley-Shannon law. The maximum channel capacity C for a system having a bandwidth B and a signal-to-noise power ration SNR2 is:
C=Blog(SNR2+1).
The SNR can be increased by designing a more accurate ADC, which can process more information in a clock cycle. The bandwidth can be increased by adding speed, i.e. designing a converter having more clock cycles in a time unit.
For an ADC with high speed and high accuracy, pipeline architecture produces the best results. In the pipeline ADC, a few relatively simple pipeline stages are connected in series. Each of these stages produces a part of the total output bits of the ADC. The pipeline architecture is efficient in terms of power dissipation. Despite the fact that the power dissipation in a base station is not as critical a parameter as it is in a mobile phone, it still has to be seriously considered.
The matching of analog components, such as capacitors and transistors, is still a problem in the integrated circuits. Any mismatch reduces the yield. The mismatch can be measured, and the error it causes can be corrected by digital calculations, if some additional circuitry is implemented. This is called calibration. In addition to digital calibration, analog and mechanical calibration methods have been developed.
The object of the present invention is provide a method for calibrating an analog-to-digital converter, which method is easy and fast to perform.
Another object of the present invention to provide an equipment for calibration of an analog-to-digital converter.
The present invention provides a method of calibrating an analog-to-digital converter, giving an initial value to a correction term of a digital value obtained in response to an analog signal supplied to the analog-to-digital converter, supplying an input to the analog-to-digital converter with an input voltage included in the voltage sector to be calibrated, said input voltage being selected such that a digital value corresponding to the input voltage is substantially of the same magnitude as a first digital value and a second digital value, supplying a first control code to the analog-to-digital converter, whereupon the first digital value is obtained as the output of the converter, supplying a second control code to the analog-to-digital converter, whereupon the second digital value is obtained as the output of the converter, calculating a residual of the voltage sector to be calibrated as a difference of the second and the first digital value, calculating a correction term related to the voltage sector, which correction term is the initial value of the correction term in the first voltage sector, by adding the correction term of the adjacent calibrated voltage sector to the residual of the voltage sector, storing in a memory the correction term related to the voltage sector, to be used in correcting the digital value obtained from the analog-to-digital converter with the correction term when the analog-to-digital converter is used to convert a signal included in the voltage sector of the voltage space from an analog into a digital form, replacing the voltage sector with the next voltage sector in the voltage space of the analog-to-digital converter, repeating the preceding seven steps until a correction term has been found for each voltage sector, reading from the converter the digital value obtained when the analog signal is converted into a digital signal, reading from the memory the correction term corresponding to the digital value, correcting said digital value with said correction term, repeating the preceding three steps during the conversion.
The present invention also provides a calibration equipment for calibrating an analog-to-digital converter, wherein the calibration equipment comprises means for giving an initial value to a correction term of a digital value obtained in response to an analog signal supplied to the analog-to-digital converter, means for supplying an input signal to the analog-to-digital converter with an input voltage included in the voltage sector to be calibrated, said input voltage being selected such that a digital value corresponding to the input voltage is substantially of the same magnitude as a first digital value and a second digital value, means for supplying a first control code to the analog-to-digital converter, whereupon the first digital value is obtained as the output of the converter, means for supplying a second control code to the analog-to-digital converter, whereupon the second digital value is obtained as the output of the converter, means for calculating a residual of the voltage sector to be calibrated as a difference of the second and the first digital value, means for calculating a correction term related to the voltage sector, which correction term is the initial value of the correction term in the first voltage sector, by adding the correction term of the adjacent calibrated voltage sector to the residual of the voltage sector, means for storing in a memory the correction term related to the voltage sector, to be used in correcting the digital value obtained from the analog-to-digital converter with the correction term when the analog-to-digital converter is used to convert a signal included in the voltage sector of the voltage space from an analog into a digital form, means for replacing the voltage sector with the next voltage sector in the voltage space of the analog-to-digital converter, means for repeating the preceding seven steps until a correction term has been found for each voltage sector, means for reading from the converter the digital value obtained when the analog signal is converted into a digital signal, means for reading from the memory the correction term corresponding to the digital value, means for correcting said digital value with said correction term, means for repeating the preceding three steps during the conversion.
The invention relates to calibration of analog-to-digital converter (ADC). In one preferred embodiment the ADC is a pipeline ADC. In the method according to the invention, the object is to split the voltage space of the ADC into input voltage sectors and find a correction term for digital output that is received as output to analog input. Then during analog-to-digital conversion the digital output can be corrected with the corresponding correction term. The calibration equipment presented by the invention can be manufactured to be inside the ADC to be calibrated or it can be external to ADC. The invention is not restricted to if the calibration equipment is internal or external.
Advantage of the present invention is that it provides a precise method for finding errors between the received and expected outputs in analog-to-digital converter. This creates basis for implementation of an high-resolution ADC.